Tripod type constant velocity joints are well known in the automobile industry as one type of constant velocity joints used in the drive system of automobiles to transfer a uniform torque and a constant speed, while operating with a wide range of joint angle. For instance, one example of the tripod type constant velocity joint was illustrated in Japanese Patent Application, S62-233522 as shown in FIGS. 1-2. This tripod type constant velocity joint typically includes tripod 15 fixed to an end of the second rotating shaft, which functions as a driven member, and hollow cylindrical housing 13 fixed to an end of the first rotating shaft 12 which functions as a drive member. Grooves 16 are formed at three locations on the inner face of the housing 13 at equal spacing in the circumferential direction and extend in the shaft direction of the housing 13.
The tripod 15 comprises a boss 17 connected to the second rotating shaft 14, and trunnions 18 each having a cylindrical shape and extending radially from three locations at equal spacing around the boss 17. Each trunnion 18 has a roller 19 fixed at a distal end of the trunnion and with needle rollers 20 engaged therein. In this arrangement, each roller 19 can freely rotate about the trunnion 18 while also be displaced in the axial direction of the trunnion 18. The constant velocity movement between the first and second rotating shafts is ensured with the rollers 19 rotatably and displaceably engaging in the grooves 16 disposed along the inner face of the housing 13. In order to facilitate the sliding movement, a pair of side faces 16a are formed in circular recesses on each side of the respective grooves 16, and each roller 19 is supported rotatably and pivotally along the side faces 16a of the grooves.
As the first rotating shaft 12 rotates, its rotational force is transmitted from housing 13, through roller 19, needle rollers 20, trunnions 18, and to the boss 17 of the tripod 15. This makes the boss 17 rotate, and which also causes rotation of the second rotating shaft 14. When the joint angle of the two rotating shafts 12 and 14 is not zero, a central axis of the first rotating shaft 12 is not aligned with that of the second rotating shaft 14, and each of the trunnion 18 displaces relative to the side face 16a of the guide grooves 16 to move around the tripod 15, as shown in FIG. 1 and FIG. 2. As a result, the rollers 19 supported at the ends of the trunnions 18 move along the axial directions of the trunnions 18, respectively, while rolling on the side faces 16a of the guide grooves 16. Such movement ensures that a constant velocity between the first and second rotating shaft is achieved.
When the first and second shafts rotate with a joint angle present, each roller 19 moves with complexity. For example, each roller 19 moves in the axial direction of the housing 13 along each of the side faces 16a of the respective guide grooves 16, while the rollers 19 change in orientation and further displace in the axial direction of the trunnion 18. Such movement of the rollers 19 cannot cause a relative movement between a peripheral outside face of each of the rollers 19 and each of the side faces 16a to be smoothly made. Thus, a relatively large friction occurs between the faces. As a result, this tripod type constant velocity joint produces three-directional axial forces as the shafts rotate. In the application of a tripod joint to the vehicles, it is known that the axial forces may cause a transverse vibration typically referred to as “shudder”, if a large torque is transmitted with a relatively large joint angle present.
In order to restrain or reduce such conventional shudder problems, various suggestions have been introduced in the art. For example, U.S. Pat. No. 6,533,668 B2 discloses a constant velocity joint construction which can reduce the shudder problem by modifying the conventional contact ellipse. As shown in FIG. 3(a), the trunnion 18′ of this joint is produced to have an elliptical cross-section when viewed from an axis normal to the trunnion shaft. As a result, the elliptical section includes a shorter diameter “B” in the length not receiving a load, and a longer diameter “A” in the length for receiving a load. This is to make a contact pattern between the inner spherical surface of the inner roller 19b and the trunnion 18′ relatively closer to a circle, when receiving a torque for a load. As a result, a longer contact diameter a′ (FIG. 3(b)) becomes smaller than the longer contact diameter of previously known constant velocity joints which have trunnions with a circular cross-section, for example, such as the trunnions shown in FR275280 and Japanese Publication No. H3-172619. However, it still has an elliptical contact pattern even though the degree of ellipse is reduced because a curvature of a longitudinal cross-section of the trunnion 18′ formed by radius r2 and R is not equal to a curvature of an axial cross-section of the trunnion 18 formed by an ellipse 18a defined as a longer diameter A and a shorter diameter B.
When these joints rotate with a joint angle present upon receiving loads, a pivotal movement of counterclockwise direction of trunnion 18′ causes a pivotal sliding action to take place on the contact ellipse. Then the pivotal sliding action operates as a frictional spinning moment so as to change a rolling direction of the roller assembly 19, which comprises the inner roller 19b and the outer roller 19a with needle bearings 20 engaged there-between. As a result, direction of the roller assembly 19 changes and it becomes in contact with inner or outer face of the guide groove 16, and thus, increasing a frictional contact force there-between. Moreover, the roller assembly 19 displaces not to parallel to the guide groove 16. Hence it is difficult for the roller assembly 19 to be smoothly rolled, and causes a significant rolling resistance.
Moreover, for manufacturing the constant velocity joint of FIG. 3, there are considerable difficulties not only to machine a complex spherical surface defined by a curvature of a longitudinal cross-section of the trunnion 18′ formed by radius r2 and R and the ellipse shape 18′ defined as a longer diameter A and a shorter diameter B, but also to measure the trunnion 18′ having a complex three dimensional profile, in terms of both inspection and quality control. These difficulties cause the contact pattern mentioned above to be inconsistent in terms of quality, which leads to high costs in manufacturing perspective and also to potential quality control issues.
Moreover, in order to provide grease-entry space for better durability and smooth operation, space “s” is provided between the upper and lower ends at the inner face of the inner roller 19b and the upper and lower portions of the spherical face of the trunnion 18′, as shown in FIG. 3(a). However, due to the relative pivotal movement of the trunnion 18′ within the open inner surface of the inner roller 19b, the grease flows out easily from the open space “s” and sufficient amount of grease cannot be retained in the open space “s” disposed between the spherical face of the trunnion 18′ and the upper and lower ends at the inner face of the inner roller 19b. As a result, the joint cannot be lubricated effectively and often suffers aggravated friction problems. This becomes more problematic when the grease is in high density condition, for example, during the initial driving stage of automobile particularly at a cold outside temperature, which causes a significant rolling resistance in the drive system.